Uranium-silicon alloy and process of producing same



Jan. 17., 1956 A, R KAUFMANN 2,731,341

URANIUM-SILICON ALLOY AND PROCESS OF PRODUCING SAME Filed July 7. 1948llIl-I l Il A-rorfnc PER mzu'r suman URAN|UM-S|L|coN PHASE DIAGRAM.

G -THERMAL ARREST ce1-AINE QNVHEATINQ.

x Two-PHASE ALLOY BY Mlcnoscomc' ExAMmATloN. I -CNE-PHASE ALLOY BYMlcRoJcaPlc EXAMINArlbN, Q EPslLoN PsRn'EcToln TEMPERATURE,

characteristics-` 4prepared as -shown in Fig. 1.

Beginning at the uranium side 'point of uranium from 1125URANIUM-SILICON ALLOY AND PROCESS OF PRODUCING SAME Albert R. Kaufmann,Lexington, Mass., assignor to the United States of America asrepresented by the United States AtomieEnergy Commission ApplicationJuly 7, 1948, Serial No. 37,408

4 Claims. (Cl. 75-134) The present invention relates to alloycompositions and,

more particularly, to novel compositions of uranium and silicon, and toa process for preparing the same. Uranium structures have thedisadvantage of being readily corroded by water. It is therefore anobject of the present invention to provide a uranium alloy that 'retainsthe good qualities of uranium metal but which at the same time isrelatively corrosion resistant.

`It is a further object of the present invention to provide novel alloycompositions containing uranium and silicon.

'It is another object of this invention to provide a new alloy of'uranium` and silicon which has low corrosion The present invention alsocontemplates the provision of. a novel uranium-silicon alloy suitablefor coating uranium.

.'ldi Patented Jan. 17, 1956 silicon, quenched from varioustemperatures. These quenching experiments have shown that the maximumsolid solubility in the gamma (y) uranium region of Fig. l is about 1.75atomic per cent silicon at 980 C. and, in the beta uranium region, lessthan 1.0 atomic per cent silicon at 750 C. The fact that the temperatureof the alpha (a) to beta solid transformation in pure uranium is notchanged by the addition of silicon indicates that the solubility ofsilicon in alpha uranium is negligibly small.

Regardless of the quenching temperature, the uraniumrich phase is alphauranium, for X-ray diffraction has A further object of this invention isto provi-de a new process for preparing uranium-silicon alloys ofexcellent corrosion characteristics.

Other objects and advantages of the invention will be 4apparent from thefollowing description taken in conjunction with the accompanyingdrawing, wherein:

Fig. l is a phase diagram of the uranium-silicon system.

According to the present invention, alloys of uranium and silicon areprepared by melting solid uranium and silicon powder in alundumcrucibles lined with beryllia, using an evacuated induction furnace. Inthe preparation of most of the alloys, the heating current can beinduced -directly in the uranium which forms the major portion of thecharge. However,when making the high 'silicon alloys it is necessary toheat the charge by heat transfer from a graphite sleeve surrounding thecrucible,

due to the high electrical resistivity of silicon.

f On the basis of thermal, microscopic and X-ray studies of the alloyswithV varying proportions of ,uranium and silicon, a completeequilibrium phase diagram has been 1500 C., while the sixth, which maybe designated as epsilon (e), decomposes by a peritectoid reaction'at amuch lower temperature, say about 930- C. In the solid state, neithervmetal issoluble in the other to any large extent andv themelting pointof either metal is lowered by the addition 'of the other.

b'e seen that'the. addition of silicon lowers themelting C. to the-eutectic temperature of 985 C. when 9 atomic per cent of silicon ispresent. The liquidus then rises steeply to 1665 C Referring to thisdiagram,

of the diagram, it can' the melting point of an alloy of the compositionUsSisr" vThe exact shape and location of the various liquidus i curveslin Fig. l are approximate.

` The -solid solubility of ysilicon in uranium has alsobeen`investig`at`ed by `'microscopic 'examinationof alloys conshown that itis not possible to retain the beta or gamma forms in these alloys byquenching. Careful examination of photomicrographs of as-melted ingotsin the composition range O to 37.5 atomic per cent silicon shows thatthe alloys formed are not in a condition of stable equilibrium `since athird phase is evident as a rim or shell around the primary dendrites.This phase is epsilon and it contains about 23 atomic per cent ofsilicon and is formed bya peritectoid reaction between gamma uranium andU5Si3 at a temperature of about 930 C. to about 945 C.

Epsilon, however, forms so sluggishly that samples 0f these alloys. fromunannealed ingots may be said to be in a state of metastableequilibrium. In fact, examination of such samples by thermal analysisand X-ray diffraction does not disclose the presence of epsilon. Thetemperature of the alpha to beta transformation in pure uranium, 665 C.,is not changed by the addition of silicon. The, temperature of ,the betato gamma transformation in pure uranium, however, is raised from 770 C.to 795 C. as shown by the phase vdiagram in Fig. l. Both of thesetransformations, the alpha to beta and the beta to gamma, are found inmetastable alloys containing up ,to 37.5 atomic per cent of silicon, asshown by the dotted lines in Fig. 1. No thermal arrest is obtained at930 C. Similarly, examination of these metastable alloys by X-raydiffraction shows only the presence of alpha uranium and U5Sis.

However,` if alloys in the range of 0 to 37.5 atomic per cent of siliconare annealed to a condition of stable equilibrium, epsilon forms and thephase relations correspond to the full lines of Fig. l. At temperaturesbelow 930 C., alloys containing less than 24 atomic per cent of siliconcontain a uranium-rich phase and epsilon, while alloys with more thanthis amount of silicon contain epsilon and U5Si3. Since stable alloyscontaining from 23 to 37.5 atomic per cent of silicon do not containany`uranium-rich phase at temperatures below 930 C the solidtransformations occur in these alloys.

The equilibrium temperature of the peritectoid reaction by which epsilonforms has been determined by using alloys with a fairly large grain sizeproduced by normal furnace cooling from the liquid state. The method fordetermination of the equilibrium temperature comprises a determinationof the temperature at which epsilon begins to form on slow cooling. Thisformation temperature is obtained by microscopic examination ofspecimens at 665 C. and 795 C. do not -of the same alloy quenched atregular temperaturel intervals during the cooling process. The firstappearance of a peritectoid rim of epsilon around particles of U5Si3 xesthe epsilon formation temperature between fairly close limits, and it ispossible to estimate the temperature at which the reaction begins fromthe thickness of the `per cubic centimeter.

ing the true equilibrium temperature of the peritectoid reaction. Usingan ailoy having an average carbon content of about 0.01 per cent byweight, photomicrographs show that, for an alloy containing 20 atomicper cent of silicon, epsilon formation begins to occur at sometemperature between about 956 C. and about 939 C. An estimated averageepsilon formation temperature for alloys containing about 3.5 to about37.5 atomic per cent of silicon is about 940 C., the temperature beingsomewhat lower in alloys ot low silicon content. It is believed thateven traces of carbon can lower the peritectoid temperature when thesilicon content is also low.

The epsilon decomposition temperature for these alloys should be thesame as the formation temperature under equilibrium conditions and, whendetermined by examination of quenched samples of slowly heatedsubstantially completely epsilonized alloy, is estimated at about 910C., which is some 30 loweithan the formation temperature determined byslow cooling. This depression of the temperature may be due to localsegregation of the small amount of carbon in the alloy, therebyproducing regions having lower epsilon decomposition temperaturesbecause of slightly higher carbon content. In general, the best Valuefor the temperature of the epsilon peritectoid reaction is judged to beabout 930 C., as shown in the phase diagram of Fig. 1.

Alloys containing epsilon are best prepared by chill casting aA melt ofthe desired composition, thus producing a line-grained structure, andthen heat treating the cast alloy at temperatures of about 750 C. toabout 850 C. to allow the reaction between uranium and U5Si3 to occur.The crystal structure of epsilon is given as tetragonal with ai=6.017 Aand 113:8.679 A. There are 1.2 uranium and 4 silicon atoms to the unitcell which would correspond to the formula UsSi. The composition ofepsilon derived from microscopic examination is 23.0 i 0.5 atomic percent of silicon (i. e., 3.40i0.l per cent of silicon by weight). Thiscomposition corresponds roughly to the formula UiuSia.

Y The determination of the phase equilibria in the range 37.5 atomic percent to 66.7 atomic per cent of silicon is complicated by the highmelting points and extreme brittleness of the phases involved. X-rayevidence shows the existence of four intermediate phases in thiscomposition range, viz., UaSis, USi, U2Si3, and USiz. The

vcompound UsSia melts sharply at 1665 C. and shows no thermal` effectsbelow this temperature. The high temperature region of the'phase diagrambetween UsSis and `USiz shows a melting point of'almost 1700"' C. rforUSiz and is known only approximately, as indicated by the dotted linesin Fig. l. USi enters into an eutectic reaction with U5S3 at about l570C. and then undergoes peritectic decomposition at about 1575* C. Oneproduct of the peritectic decomposition of USi is UzSia, which in turnundergoes peritectic decomposition at about l6l0 C. One kproduct of theperitectic decomposition of U2Si3 is U'Siz which melts at a temperaturein the neighborhood of 1700 C. At the silicon end of the system USiaforms by a peritectic reaction at about l0 C. and enters into a eutecticreaction with delta the siliconrich solid solution, at about l3l5 C.,the melting point of the silicon employed being 1420 C.

USis has the CusAu type of crystal structure, being simple cubic withuranium atoms at 000 and silicon atoms at 011/52,'11/20 and 1/201/2. Thelattice parameter is 4.03 Angstrom units, the calculated density 8.23grams per cubic centimeter and the measured density 8.11 grams rlhecompound USig decomposes peritectically at 15l0 C. to give USig and aliquid containing about 82 atomic per cent of silicon. USis and deltasilicon form a eutectic alloy melting at about 1 315 C. at a compositionof about 86 atomic per cent silicon.

To summarize, the uranium-silicon alloy system is composed ofthefollowing solid phases:

(1) Alpha uranium, stable up to 665 C., and containing a negligibleamount of silicon in solid solution;

(2) Beta uranium, stable between 665 C. and 770-795 C., depending oncomposition, and containing less than 1 atomic percent of silicon insolution;

(3) Gamma uranium, stable between 770-795 C. and 1125" C., its-meltingpoint, which forms an eutectic with U5Si3 at 985 C. and contains about1.75 atomic per cent of silicon in solid solution at this temperature;

(4) Epsilon, which. formsby a peritectoid reaction between gamma uraniumand U5Si3 at 930 C. and contains 23 atomic per cent silicon;

(S) U5Si3, which melts at 1665 C. and forms an eutectic with USi at l570C.;

(6) USi, which forms by a peritectic reaction between liquid and U2Si3at l575 C.;

(7) UzSia, which forms by a peritectic reaction between liquid and USizat 1610 C.;

(S) USia, which melts at approximately l700 C., and has a body-centeredtetragonal crystal structure;

(9) USia, which forms by a peritectic reaction between USi2 and liquidat 1510 C., and which forms an eutectic with delta silicon at 13l5 C.;and

(10) Delta silicon, which contains very little uranium in solid solutionand melts at 1420 C.

Uranium-silicon alloys containing more than 30 atomic per cent ofsilicon are extremely brittle and easily broken. Because of this alloyscontaining more than 30 atomic per cent of silicon are of comparativelysmall utility.

The most useful of the binary uranium-silicon alloys are thosecontaining from 15 to 25 atomic per cent of silicon which have been heattreated so that a high proportion of the allo; has been converted to theepsilon phase. Epsilon is an intermediate phase in the uraniumsiliconsystem 4containing about 23 atomic per cent of silicon. This phase formsby means of a peritectoid reaction. whose temperature is lowered by thepresence of carbon. Investigation of the corrosion resistance of epsilonin various media shows that epsilon is quite superior to'pure uraniummetal and many other uranium alloys. The epsilon phase is a highstrength alloy which possesses some degree of ductility.

In forming a binary uranium-silicon alloy containing fromV 15 to 25atomic per cent of silicon and having a high proportion of the alloy inthe epsilon phase, it. is important to convert the melt to an alloyhaving a fine grain size. Such a tine-grained alloy may be produced bychill casting the melt into a copper or a graphite mold. The alloyconstituents, uranium and silicon, are melted in a graphite Crucible,arranged for rbottom pouring, in an evacuated induction furnace. Whenthe alloy is completely molten and well mixed, it is poured through thebottom of the crucibley into a copper mold. The temperature of thecopper mold is `not sensibly above that of theroonn 'so that a chillcasting in vacuo is obtained, giving an alloy in the desiredtine-grained condition. Very fine-grained alloys may also be obtained bya liquid quench technique in which the completely molten alloy isquenched immediately'in cold water.

This tine-grained alloy should then be annealedin vacuo or in an inertatmosphere'for at least 10 hours at a temperature between 700 C. and850. C. To insure that p conversion to the. epsilon phase issubstantially complete,

it is ordinarily desirable to 'continue the heat treatment -for at leastapproximately 1.6 hours. The optimum temperature' range for annealingruns from 750l C. to 800 C. For alloys in which the presence of smallamounts of carbon as an impurityrhas lowered the peritectoid temperatureat which the epsilon phase forms, it is preferable to carry out theannealing at temperature below 800 C. Since the epsilon phaseissubstantially softer. than the corresponding cast structure, .it ispossible to :make use of 75.,- har'dness measurements in studying` theformationV ofthe epsilon phase which takes place during the annealingoperation. The epsilon phase forms by a peritectoid reaction between theuranium matrix and UsSis. The production of the epsilon phase in analloy results inan increase in its density. VThe amount of epsilon phaseproduced reaches a maximum for alloys containing from 20 to 25 atomicper cent of silicon.

Binary uranium-silicon alloys which contain from to 25 atomic per centof silicon and which have been heat treated to convert a high proportionof the alloy to the epsilon phase have corrosion resistant propertieswhich are far superior to those of metallic uranium. Structures madefrom such alloys stand up much better than uranium structures when theyare placed in direct' contact with water. Structures made from theepsilon alloy appear to be from 500 to 1,000 times more `corrosionresistant in boiling water containing 2 parts per million of chlorideion than similar structures made from pure uranium metal. Uraniumcorrodes quite rapidly in distilled water at 100 C. Binaryuranium-silicon alloys containing from 15 to 25 atomic per cent ofsilicon which have not been heat treated to transform a high proportionof the alloy to the epsilon phase corrode almost as rapidly as pureuranium in distilled water at 100 C. This is probably due to the factthat the chill cast alloy which has not been heat treated has a uraniummatrix which is very susceptible to corrosion. The binaryuranium-silicon alloys which have been heat treated possess a continuousmatrix of the epsilon phase. The binary uranium-silicon alloyscontaining from 15 to 20 atomic per cent of silicon which have been heattreated to transform them into the epsilon phase corrode only slightlyduring the iirst 100 hours in water at 100 C., and thereaftersubstantially no corrosion occurs even though the test is continued for500 hours. Similar binary alloys containing from to atomic per cent ofsilicon show substantially no corrosion in distilled water at 100 C.even after 500 hours. In order to obtain good corrosion resistance inthese alloys, it is important to anneal the chill cast alloys for aminimum of 16 hours in order to secure substantially completetransformation of the alloy to the epsilon phase. The corrosion of thesealloys in water does not seem to be appreciably atfected by the presenceof 2 to 5 parts per million of chloride ion in the water or by bubblingair, hydrogen, or oxygen through the water for long periods of time. Afilm or coating of an oxide nature apparently forms on epsilon alloysduring corrosion in water and serves to protect the alloy from furthercorrosion. No noticeable galvanic corrosion occurs when epsilon alloysare placed in contact with silver, aluminum, beryllium, copper, lead,zinc, tin, nickel, monel or stainless steel in boiling distilled water.These epsilon phase alloys have been tested in a steam autoclave at apressure of 125 pounds per square inch (178 C.) and have been found tocorrode only slightly faster under these conditions than in boilingdistilled water. Carbon impurities have no effect upon the corrosionresistance of the epsilon phase alloys.

Controlled tests were run in which solid cylinders of pure uranium andof an epsilon phase binary uraniumsilicon alloy were placed in aluminumjackets which were pinholed and heated in a steam autoclave at apressure of 125 pounds per square inch. The uranium cylinder failedafter 2 hours while the cylinder of epsilon phase alloy showed nonoticeable changes after 16 days of treatment in the autoclave.

A solid cylinder of epsilon alloy containing 15 atomic per cent siliconwas completely enclosed in an aluminum jacket. A #80 hole was drilledthrough the jacket to produce a leak. This cylinder was found to swellonly about 5 mils on the diameter after 90 days in boiling water;whereas a similarly tested cylinder of uranium developed swellings 80mils high in three days. Likewise, a jacketed cylinder of pure epsilon(about 25 atomic per cent silicon) when pin-holed and tested in steam at178 C. for 31 days showed only some localized swelling while a similarlytreated cylinder of uranium failed in two hours.

A 25 atomic per cent silicon'specimn of epsilon appears to resistoxidation at 200 C. in air for an indefinite time while pure uraniumoxidizes quickly in air at 100" C. A similar specimen of epsilon alloyis not attacked by hydrogen at 325 C. after 4 hours of heating whilepureuranium disintegrates into a ne powder in 20 minutes at 225 C. inhydrogen.

The density of binary uranium-silicon alloys' containing from 15 to 25atomic per cent of silicon which have been heat treated to convert thealloy to the epsilon phase decreases from a value somewhat greater than16.5 grams per cubic centimeter at 15 atomic per cent silicon to a valuesomewhat less than 15.5 grams per cubic centimeter at 25 atomic per centsilicon. The density of th'ese binary alloys as cast is slightly lessthan the corresponding alloy after heat treatment to convert the alloyto the epsilon phase. An alloy containing 23 atomic per cent of siliconwhich has been converted to the epsilonphase has a density of 15.45grams per cubic centimeter.

Whereas binary uranium-silicon alloys containing from 15 to 25 atomicper cent of silicon as cast have a hardness of 40 or greater on theRockwell C scale, the same alloys after heat treatment to convert themto the epsilon phase are much softer and have a hardness between 25 and30 on the Rockwell C scale.

Stress-strain data indicate that the yield point in compression forbinary uranium-silicon alloys containing from 15 to 25 atomic per centof silicon which have been heat treated to convert the alloy to theepsilon phase appears to be about 125,000 pounds per square inch and isapparent after a 0.5 to 1 per cent reduction in length. The ultimatestrength lies roughly at about 275,000jfpounds per square inch, and thetotal reduction in length prior to breaking is between 12 and 16 percent. Alloys with the lowest silicon content have the highest yieldpoint and the greatest over-all plasticity. Cast uranium-silicon alloyswhich have not been heat treated to convert them to the epsilon phasehave a high yield point but are comparatively brittle and fail with verylittle deformation.

Hot compression data on an epsilon phase alloy containing 23 atomic percent of silicon show that the yield point and compressive strengthdecrease rapidly from 600 C. to 850 C. The yield strength at 600 C. is55,000 pounds per square inch, and it decreases to 18,000 p. s. i. at700 C. The yield strengths at 750 C. and at 850 C. are 10,000 p. s. i.and 4,000 p. s. i. respectively. The lowvalues of compressive strengthin the 750 to 850 C. range show that the epsilon alloy has a fair amountof ductility at these temperatures.

A binary alloy of uranium and silicon which has been heat treated toconvert it to the epsilon phase may be extruded if care is taken tocompletely sheath the billet of epsilon alloy in copper tubing. Thecopper sheath serves to lubricate the die and to prevent any oxidationof the epsilon billet. It is important to carry out the extrusion at atemperature lying in the range of 750 to 800 C. At temperatures lowerthan about 750 C. the epsilon alloy possesses insuicient ductility to bereadily extruded. At temperatures greater than 800 C. there is atendency for the epsilon alloy to revert back to a iinely dividedmixture of uranium phase and U5Si3. Since the peritectoid temperature atwhich the epsilon phase is in equilibrium with gamma uranium and UsSisis markedly lowered by the presence of small amounts of carbonimpurities in the alloy, it is necessary to rigidly control thetemperature of extrusion so that the epsilon alloy will be sutiicientlyductile but not heated above its decomposition temperature. Epsilonalloys are soft enough to be threaded and machined in various otherways.

Although the present invention has been described with respect toparticular illustrative examples and embodiments, it will be understoodthat variations and modifications may be made and equivalentssubstituted therefor 'ii' withoutiirlepartiuggfrom Athe vprinciples andtrue vspirit of theA invention. Suchl lvariations and modilications arebelieuedftohewithinthe scope'of the present spec'rcation andwithinthepurview .of .therappended claims.

VI claim:

1. A process` of making asoft, ductile and corrosion resistant ,article:from a binary alloy of vuranium and silicony containing from 15 lto .25yatomic per cent of silicon which comprises chill casting the articlefrom a -melt of said alloy vand then heat-treating the cast article for:at least ten hours at a temperature between 70" and`=850 v V2'. .The:epsilon .phase of `a binary valloy of uraniumvand silicon:characterized ashaving a tetragonal crystal structntexwith a1==.6.0l.7Ay and Aa3=8.679 ,A and having l2 uraniumand '4Afsiliconatoms -to .theunit ceil, Iand having Aa'silicon:contentziof 23.0ci 'atomic per cent,and formed by a peritectoid ,reaction between gamma uranium and UsSia at930 C. l

3. A corrosion resistant binary allow of uranium rand silicon containingfrom l5 to 25 atomic percent of silicon wherein the normally brittleintermetallic compounds of uranium and silicon are present in therelatively soft and ductiie epsilon phase.

4. A corrosion resistant binary alloy of uranium and silicon containingfrom 2G to 25 atomic percent of silicon wherein the normally brittleintex'rnetellic compounds of uranium and silicon are present in therelatively soft and dnctile epsilonphase.

References Ciedin the le of this patent Hensen: Der Aufbau derZWeistoff-legierungen, page i672. Edward Brothers, Inc., 1943.

1. A PROCESS OF MAKING A SOFT, DUCTILE AND CORROSION RESISTANT ARTICLEFROM A BINARY ALLOY OF URANIUM AND SILICON CONTAINING FROM 15 TO 25ATOMIC PER CENT OF SILICON WHICH COMPRISES CHILL CASTING THE ARTICLEFROM A MELT OF SAID ALLOY AND THEN HEAT TREATING THE CAST ARTICLE FOR ALEAST TEN HOURS AT A TEMPERATURE BETWEEN 700* AND 850* C.
 3. A CORROSIONRESISTANT BINARY ALLOW OF URANIUM AND SILICON CONTAINING FROM 15 TO 25ATOMIC PERCENT OF SILICON WHEREIN THE NORMALLY BRITTLE INTERMETALLICCOMPOUNDS OF URANIUM AND SILICON ARE PRESENT IN THE RELATIVELY SOFT ANDDUCTILE EPSILON PHASE.